Production of asymmetrical microporous membranes by double irradiation and asymmetrical membranes

Abstract

Process for the production of asymmetrical microporous membranes by double irradiation. A membrane (5) is exposed to two irradiations, whereof one has an inadequate energy level to pass through the same, whereas the other passes through membrane (5). The irradiations produce molecular defects, which are attacked or etched in preferred manner by certain chemical agents. Thus, two networks of pores are obtained, one network being of large diameter and of a non-issuing nature (26), the other being smaller (27, 28, 29) and which issue on to the two surfaces (31, 32) of membrane (5). Application to filtration and sterilization.

We claim: 
     
       1. A process for making an asymmetrical microporous membrane defined between a first surface and a second surface, which comprises: (a) first irradiating said membrane by using particle beams having sufficient energy only to penetrate a partial distance into said membrane in order to form a first set of pores which penetrate partially through said membrane;   (b) second irradiating said membrane by using particle beams having sufficient energy to pass through the entire membrane in order to form a second set of pores which penetrate entirely through said membrane; and   (c) etching said membrane by chemical means for obtaining a first network of said first set of pores and a second network of said second set of pores at the locations traversed by first and second irradiations of said membrane thereby to issue said first set of pores of said first network onto said first surface of said membrane only and said second set of pores of said second network onto both said first surface and second surface of said membrane; wherein at least a part of said second set of pores being issued on said second surface and on said first set of pores, the diameters of said first set of pores of said first network being larger than the diameters of said second set or pores of said second network.   
     
     
       2. The process according to claim 1, wherein etching said membrane comprises a first and a second etching, the first etching creating the first network of said first set of pores and the second etching creating the second network of said second set of pores, the first irradiation, first etching, second irradiation and second etching being carried out sucessively. 
     
     
       3. The process according to claim 1, wherein said first and second irradiations are carried out with heavy ions issued from a particle accelerator. 
     
     
       4. The process according to claim 1, wherein said particles used to penetrate said membrane in said first irradiation have a first incidence relative to said membrane and said particles used to penetrate said membrane in said second irradiation have a second incidence which differs from said first incidence. 
     
     
       5. The process according to claim 2, wherein said first and second irradiations are carried out with heavy ions issued from a particle accelerator. 
     
     
       6. The process according to claim 2, wherein said particles used to penetrate said membrane in said first irradiation have a first incidence relative to said membrane and said particles used to penetrate said membrane in said second irradiation have a second incidence which differs from said first incidence. 
     
     
       7. The process according to claim 2, wherein said particles used in said second irradiation have several incidences relative to said membrane. 
     
     
       8. An asymmetrical microporous membrane defined between a first surface and a second surface, which comprises a first network of pores issuing on said first surface only, and a second network of pores, at least a part of pores of said second network, issuing on said second surface and pores of said first network, the pores of said first network having a larger diameter than the pores of said second network. 
     
     
       9. The membrane according to claim 8, wherein said pores of said first network are directed differently than the pores of said second network. 
     
     
       10. The membrane according to claim 9, wherein said pores of said second network are directed in several directions.